It is generally known that reducing the exposure to oxygen of oxygen sensitive articles maintains and enhances the quality and shelf life of the article. For instance, reducing the oxygen exposure of oxygen sensitive food products in a packaging system maintains the quality of the food product and avoids food spoilage. Foods, beverages, pharmaceuticals, medical devices, corrodible metals, analytical chemicals, electronic devices, and many other products may perish or experience diminished shelf life when stored too long in the presence of oxygen. Reduced oxygen exposure may help keep the product in inventory longer, thereby reducing costs incurred from waste and having to restock.
Manufacturers of packaging materials have developed packaging materials and systems to limit and/or control the amount of oxygen to which a packaged article may be exposed. Such materials and methods may include packaging articles in a package environment, or “headspace”, with reduced oxygen levels. Modified Atmosphere Packaging (MAP) and vacuum packaging are two methods that are commonly used to control the amount of oxygen in a package. MAP involves the modification of the head space gas in a package in order to prolong the shelf life of the product it contains. In some MAP applications, the headspace may have substantially no oxygen. In other MAP applications, the headspace may have a predetermined level of oxygen. The success of MAP generally depends on the ability to control the concentration of oxygen within the package. In vacuum packaging, the atmosphere may be substantially removed so that the package environment is substantially free of oxygen.
In MAP applications for meat products, the raw meat may be packaged in a low level oxygen (O2) environment. Packaging systems having low levels of oxygen are desirable because the fresh quality of meat can generally be preserved longer under anaerobic conditions than under aerobic conditions. Maintaining low levels of oxygen minimizes the growth and multiplication of aerobic bacteria. One example of a modified atmosphere environment is a mixture of gases consisting of about 30 percent carbon dioxide (CO2) and about 70 percent nitrogen (N2). Typically, low oxygen packaging environments may provide an atmosphere that helps prevent or inhibit excessive metmyoglobin (brown) formation in red meat products. In some MAP applications, it may be desirable to maintain the oxygen level at a predetermined concentration.
Another method of reducing oxygen exposure is to incorporate an oxygen scavenging composition into the packaging structure, such as in a film or tray. Oxygen scavenging compositions are compositions that consume, deplete, or reduce the amount of oxygen in a given environment. There are a wide variety of different compositions that can be used in oxygen scavenging applications. Exemplary compositions are described in U.S. Pat. Nos. 5,211,875; 5,350,622; 5,399,289; and 5,811,027 to Speer et al. and WO 99/48963 to Cai et al. The oxygen scavenging compositions can be “triggered” by exposing the composition to a radiation source, such as actinic radiation, having sufficient power for a sufficient amount of time to initiate oxygen scavenging.
Methods of triggering oxygen scavenging compositions typically use low-pressure mercury germicidal lamps that have an intensity output from about 5 to 10 mW/cm2. These lamps are commonly referred to as germicidal since the principal emission is at 254 nm. A dosage of UV-C light between about 100 to 1600 mJ/cm2 is typically needed to trigger oxygen scavenging. For details on preferred methods for activating such oxygen scavenging compositions at point of use, see Speer et al., U.S. Pat. No. 5,211,875, Becraft et al., U.S. Pat. Nos. 5,911,910, and 5,904,960, and co-pending applications U.S. Ser. No. 09/230,594 filed Aug. 1, 1997, and Ser. No. 09/230,776 filed Jul. 29, 1997, and U.S. Pat. No. 6,233,907 (Cook et al.), all of which are incorporated herein by reference in their entirety.
Unfortunately, oxygen scavengers do not always activate on command. This may result from a number of factors, including defective scavenger compositions, inadequate triggering conditions, operator error, or a combination of these or other factors. In many instances, it may not be readily apparent whether the oxygen scavenging composition is defective or whether the failure originated in the triggering equipment. Typically, conventional oxygen scavengers do not themselves visually indicate whether or not they are active. In response to this uncertainty, operators of packaging assembly plants prefer to verify scavenger activity as soon as possible after triggering. The longer a failed triggering attempt remains undiscovered, the more waste and expense is incurred, especially where packaging equipment operates at high speeds.
In addition, defective seals or openings in the packaging may permit oxygen to enter into the headspace within a package. Such defective packages may not be easily discernable. As a result, a packaged article may be exposed to an undesirable level of oxygen, which may result in loss of shelf-life or spoilage.
There are several methods for verifying oxygen concentration in a package. Prior art methods for verifying oxygen scavenger activity in a low oxygen package involve detecting oxygen concentrations in the package headspace. Oxygen concentrations are typically measured after the package has been assembled and equilibrium of oxygen levels established among the headspace, package layers, and package contents. Detection of sufficiently reduced oxygen levels within the headspace allows one to determine if the package has maintained a low oxygen atmosphere and to infer whether an oxygen scavenging compound has been successfully activated.
Under this approach, one typically has two options, neither of which is particularly satisfactory. One option is to leave an oxygen indicator in the package headspace after it has been assembled and sealed. For example, Mitsubishi teaches an indicator comprising glucose and methylene blue, encased within a sachet. The sachet is left inside the package after it is sealed. A color change within the sachet indicates the presence of unwanted oxygen.
This approach has several disadvantages, however. Sachets must be attached to the package to avoid their being accidentally ingested by the consumer. Some package contents require a moisture-free storage environment. Yet, in the case of the Mitsubishi glucose/methylene blue indicator, moisture may be required to produce a color change. Also, sachets potentially introduce contaminants or other substances into the package that may be incompatible with its contents or accidentally ingested. For some applications, manufacturers may not want to leave indicators in packages where consumers may misinterpret the information the indicator provides.
Another option is to use probes to measure the gas content within the headspace. One commonly used headspace gas analyzer is available from Mocon, Inc. Unfortunately, the use of probes that rely on gas chromatography and other such analytical techniques typically requires removing a sample of the atmosphere within the package. This technique invariably requires some sort of device that will penetrate the package and remove a portion of the gas within the headspace. The device inevitably leaves a hole in the package, destroying the integrity of the package. As a result, this may require sacrificing the sampled package.
Additional methods of measuring oxygen concentration include the use of luminescent compounds that may be incorporated into the film lidding or into an interior space of the package itself. When exposed to light at a proper wavelength, the molecules of the luminescent compound can absorb energy which may cause electrons to move from a ground state energy level into an excited state energy level. From here, the excited molecules relax back to the ground state through a process known as vibrational relaxation. In vibrational relaxation, the absorbed energy is transferred to surrounding molecules through molecular collisions.
Alternatively, the molecule may relax to the ground state by emitting a photon. In some molecules, the electron may move from a high energy singlet state into a high energy triplet state before emitting a photon and returning to the ground state. A transition from the high energy singlet state is called fluorescence. Fluorescence transitions have a relatively short life, on the order of 10−8 to 10−4 seconds. Transitions from the triplet state to the ground state are called phosphorescence. Phosphorescence transitions are relatively longer than fluorescence transitions and may be on the order of 10−4 to 10−2 seconds.
Both fluorescence and phosphorescence transitions are quenched by oxygen. In 1919, Stern and Volmer reported that oxygen quenches the luminescence of certain compounds. From their experiments, they determined that the quenching-related decrease in luminescent intensity or lifetime of the excited state may be correlated to the oxygen concentration. This correlation may be expressed by the Stern-Volmer equation:
                                          F            0                    F                =                                            τ              0                        τ                    =                      1            +                                          k                q                            ⁢                                                τ                  0                                ⁡                                  [                  Q                  ]                                                                                        (        1        )            Wherein:F0 is the intensity of the luminescence in absence of oxygen;F is the intensity of the luminescence in presence of oxygen;τ0 is the lifetime of the excited state in the absence of oxygen;τ is the lifetime of the excited state in the presence of oxygen;kq is the bimolecular quenching constant; and[Q] is the concentration of oxygen.
A plot of F0/F or τ0/τ versus [Q], also known as a Stern-Volmer plot, is expected to provide a linear plot because F0/F and τ0/τ are generally linearly dependent on the oxygen concentration. A plot of the Stern-Volmer equation includes a y-intercept of 1 and a slope of kqτ0, which is also referred to as the Stern-Volmer constant K. From the Stern-Volmer plot, the concentration of oxygen may be deduced by measuring the intensity of the luminescence or the lifetime of the excited state (τ). This relationship has been used in the prior art to determine the oxygen concentration. However, both the intensity of the luminescence and the lifetime of the excited state are a function of temperature as well as oxygen concentration. Both luminescence intensity and the lifetime of the excited state will change at varying temperatures. As a result, prior art devices utilizing the Stern-Volmer equation to determine oxygen concentration have been limited to isothermal conditions. Measuring the intensity or lifetime at temperature conditions that are different from the initial Stern-Volmer plot may produce results that are inaccurate and do not reflect the actual oxygen concentration. Accordingly, there still exists a need for a non-invasive method and device that may be used to accurately measure oxygen concentrations under various temperature conditions.